An optical disk has a structure that an information recording/reproduction layer is formed on a substrate. The substrate used in the optical disk is manufactured by injection compression molding an aromatic polycarbonate resin (may be abbreviated as PC hereinafter). In this method, a molten resin is injected into a mold in which a stamper as a master having engraved irregularities for forming pits, grooves or lands is installed so as to transfer the irregularities of the master to the surface of the resin, and the substrate is taken out by opening the mold after pressing and cooling steps.
In this method, a failure such as the collapse of the transferred irregularities as shown in FIG. 1 may occur. This is caused by contact between the irregularities of the stamper and the irregularities of the substrate in the step of removing the substrate from the mold. Since a deformed portion of the irregularities looks white and cloudy, it is called “cloud”. A product having this cloud is judged as falling short of the appearance quality standards and not accepted. Further, a read signal and a tracking signal designed by using the master cannot be obtained due to the collapse of the irregularities, and the deterioration of the characteristic properties of the optical disk such as a jitter or tracking error occurs. Therefore, high releasability is required to manufacture an optical disk substrate.
Meanwhile, optical disks are divided into conventional optical disks such as CD's (track pitch of 1.6 μm) and high-density optical disks such as DVD's (track pitch of 0.74 μm), Blu-ray disks (track pitch of 0.32 μm) and HD DVD's (track pitch of 0.40 μm). The high-density optical disks have a track pitch of 0.74 μm or less which is narrower than that of the conventional optical disks. Therefore, to mold a high-density optical disk, higher releasability is required.
To improve the releasability of an optical disk substrate at the time of molding, various measures are conceivable. For example, patent document 1 discloses a method in which a fatty acid film is formed on a stamper before molding to reduce bonding strength between the stamper and a resin. However, this method has a drawback that when the number of cumulative shots increases, the film falls off and the above effect is not obtained.
Patent document 2 discloses a method in which an air blow is optimized to suppress a release failure caused by the nonuniformity of the air blow as auxiliary releasing means. However, a release failure is not always caused by the nonuniformity of the air blow and the effect has its limits.
To improve releasability, a release agent to be blended with PC is also under study. For studies on a release agent used to mold a high-density optical disk substrate, molding conditions must also be taken into consideration. That is, to mold a high-density optical disk substrate having a fine structure, the flowability of PC must be raised. To this end, molding must be carried out at about 380° C. which is higher than the molding temperature (340° C.) of a conventional optical disk substrate. Therefore, to mold a high-density optical disk substrate, a release agent suitable for the above molding conditions must be used.
Patent document 3 proposes that PC used for an information recording medium such as an optical disk should be blended with an aliphatic ester having an alkali metal ion content of 100 ppm or less to improve its releasability and hydrolytic resistance. However, this proposal is for a conventional optical disk which is molded at about 340° C., and this technology cannot be applied to a high-density optical disk directly. To mold a high-density optical disk substrate, further studies must be made on the type, amount and alkali metal ion content of the aliphatic ester.
Patent document 4 proposes that PC should be blended with a glycerin monoester having a sodium content of 20 ppm or less to improve its releasability and residence heat stability when it is used as a general molding material. However, releasability in this proposal is the releasability of a general molded product, and this technology cannot be always applied to the molding of an optical disk substrate directly. This proposal merely suggests the molding of PC having a high viscosity average molecular weight which is not suitable for the molding of an optical disk substrate at about 340° C.
Patent document 5 proposes that the amount of the residual sodium contained in PC having a low content of a terminal hydroxyl group should be reduced to 1 ppm or less or a fatty acid monoglyceride should be contained in an amount of 20 to 5,000 ppm in order to prevent the production of a white point on an optical disk at a high temperature and a high humidity. However, as releasability is not taken into account at all and a conventional optical disk which is molded at about 340° C. is targeted in this proposal, this technology cannot be applied to the molding of a high-density optical disk substrate directly.
Patent document 6 proposes that a resin having a low content of a volatile component should be used to reduce the amount of a deposit on a stamper and obtain an excellent high-density optical disk.
(Patent Document 1) JP-A 09-306038
(Patent Document 2) JP-A 2000-207788
(Patent Document 3) JP-A 7-192309
(Patent Document 4) JP-A 10-168296
(Patent Document 5) JP-A 3-100501
(Patent Document 6) JP-A 2000-129113